1. Field of the Invention
This invention relates to a process for making alkenyl aromatic polymer rigid foams having a wide range of cell morphologies by utilizing nano-particles as nucleating agents. Such rigid foams are useful for forming rigid insulating foam boards suitable in many conventional thermal insulation applications.
2. Description of the Related Art
The physical properties of rigid polymer foam boards, such as their compressive strength, thermal conductivity, dimensional stability, water absorption rate, depend in large part on the micro-structure of the material forming the boards, i.e., the cell morphology of the foam. However, it can be difficult to control polymer foaming to the degree necessary for consistent production of a desirable cell morphology that will tend to optimize the overall foam properties, or to improve a specific property, such as the thermal insulation value of the foam.
Prior art attempts to make foam micro-structures having desirable cell morphologies have included the use of nucleation agents such as powders formed from inorganic oxides, various organic materials and metals. Among these nucleation agents, the inorganic oxides, such as talc, titanium dioxide and kaolin, are the most commonly used. The size, shape, particle distribution and surface treatment of the nucleation agent(s) utilized to form a foam will all tend to affect the nucleation efficiency and, consequently, the cell size morphology and distribution in the resulting foam.
Conventional methods for controlling the cell morphology, however, tend to be limited by difficulties in evenly distributing particles of the nucleation agent throughout the polymer and/or suppressing agglomeration of the dispersed particles. Certain structural defects in the resulting foams are generally attributed, at least in part, to dimensional differences between the particles of the nucleating agents—which may be in the range of several microns, particularly in situations where there has been some degree of agglomeration—and the desired cell microstructures—which may have a target cell wall thickness of one micron or less—for a low density commercial insulation foams.
This size difference between the nucleation agent particles and the cell wall thickness may also result in relatively weak interactions between the nucleating agent and nano-scale polymer, thereby weakening the overall foam structure. Similarly, cell defects may also be attributed, at least in part, to the hydrophilic surface of most conventional inorganic nucleation agents that makes them difficult to disperse evenly in a polymer. These effects tend to result in processing difficulties, such as corrugation of the resulting foam board, when nucleation agents are added at levels greater than about 2 weight percent or the median cell size of the resulting foam is less than around 120 microns.
Prior art attempts to avoid foam structure corrugation effects have utilized cell size enlarging agents such as the waxy compositions disclosed in U.S. Pat. No. 4,229,396, the contents of which are hereby incorporated by reference in their entirety, and the non-waxy compositions disclosed in U.S. Pat. No. 5,489,407, the contents of which are hereby incorporated by reference in their entirely.
Another effort directed toward foam structures having bi-modal cell morphology (Kanelite Super EIII, Kaneka, Japan) included use of immiscible blowing agents, such as water and hydrocarbon. This combination, however, tends to result in processing difficulties due to the low solubility of water in the polymer and the reaction of water with fire retardant, such as hexabromocyclododecane (HBCD) at the elevated temperatures typically utilized during the extrusion process.